The present invention has application to the control of all fluids which require a reduction of fluid flow or a shut-off fluid flow due to critical temperature changes in the fluid. The invention has particular application in the construction of gas analyzers which use heated sampling systems to prevent liquid condensation in the system and which systems draw a gas through the system by using an aspirator to affect the gas flow. It is desirable in such devices to interrupt the aspiration of gas flow when the gas sample temperature falls below the dew point causing condensation within the apparatus. Known devices use a temperature sensor and a solenoid valve to control the aspirating air stream by shutting it on and off. Such an arrangement involves expensive electrical circuitry and connections as well as piping which frequently limits its applicability due to ambient effects on the valve and electrical components.